The present invention relates to a semiconductor chip package, and more particularly, to a package of a micro-electro-mechanical systems (MEMS) device and a method for fabricating the same.
In general, MEMS technology is the integration of very small mechanical devices installed inside a semiconductor chip such as sensors, valves, gears, reflecting mirrors, and drivers on a computer. Thus, MEMS are often called intelligent machines. MEMS usually comprise micro-circuits integrated on very small silicon chips installed with very small mechanical devices such as reflecting mirrors and sensors.
MEMS technology recently receives great attention because MEMS technology allows manufacturing of electronic parts using micro-electronic technology under the same cost-effectiveness. Also, MEMS technology becomes the basis of a next generation integration technology, and is considered critical to provide a ubiquitous environment. For this reason, the Korean government set the ‘IT839’ policy based mainly on the MEMS technology. According to the MEMS technology, instead of fabricating micro-actuators and sensors one by one for each time, several hundreds of micro-actuators and sensors can be simultaneously fabricated on a silicon wafer. The known silicon chip fabrication technology can be directly applied to the MEMS technology. Thus, a numerous number of MEMS devices can be fabricated on a silicon wafer directly using the known semiconductor fabrication processes.
As MEMS technology has been progressively improved, product classes are standardized. Thus, for fabrication processes, product designers can also focus on the design rule, which is based on methods generally used in most of the electronic products.
However, a packaging cost of MEMS devices (e.g., sensors) reaches about 30% to 70% of the total cost. This high cost may impede the commercialization of MEMS unless high-performance and low-cost packaging be achieved. In an attempt to reduce the packaging cost for sensors and others, wafer level chip scale packaging (WLCSP) that allows the implementation of mass-production processes is being vigorously applied. The WLCSP is often considered as one of the critical processes not only in the MEMS technology but also in the typical system in package (SiP) technology. The implementation the WLCSP technology requires developing bonding technology.
Bonding technology which has been currently implemented will be described hereinafter.
First, an anodic bonding method bonds glass (e.g., Pyrex) on a silicon wafer. When a voltage is applied to both ends of the glass at an ascending temperature, a Na2O component inside the glass is ionized, so that positive ions of Na+ are moved to a negative pole, and negative ions of O2− form a layer of charge on the side of a positive pole. Strong electrostatic power is generated between the charge layer of the negative ions and an electrode of aluminum (Al), and a strong bonding is created between the glass and the silicon wafer due to an interfacial chemical reaction. In a wafer level process, while a voltage of 300 V to 1000 V is applied at a temperature of 300° C. to 500° C., the glass is heated and cooled repeatedly for about 3 minutes to 10 minutes so as to create the bonding. At this time, a bonding force is not required greatly.
The above described bonding method is not sensitive to surface roughness, wafer bowing, and particles, and can simply create the bonding in a clean environment because a medium material or a buffer layer is not interposed between the substrate and the target (e.g., glass). Also, the bonding is possible under the condition of applying a voltage less than 1,000 V and a temperature below 400° C. The bonding can also occur between various materials such as metal and glass, glass and glass, and silicon and glass. Moreover, since glass is used as a packaging material, the inside of externally fabricated devices and operation thereof can be visually observed. Since the inside of the devices bonded together is usually hermetic to a vacuum (i.e., airtight characteristic), the bonding method can be applied to a packaging of various vacuum devices. The bonding method can also be implemented to form multiple-layer structures. Thus, the bonding method can be applied to fabricate various three dimensional MEMS. In other words, the bonding method has a wide range of applications. In particular, this type of packaging is biologically compatible, and thus, it can be applied to sensors for medical purposes.
However, this bonding method may have poor compatibility with complementary metal-oxide semiconductor (CMOS) devices because of accumulated alkali ions. Particularly, glass that is applied for the bonding includes sodium (Na), which is one incompatible component in semiconductor fabrication processes, the bonding method may not be applied to the whole semiconductor fabrication process. While the fabrication processes proceed, O2 is likely to be desorbed. As a result, the inner pressure of cavities tends to increase.
According to a fusion bonding method or a silicon direct bonding (SDB) method, two silicon wafers to be bonded together are aligned with each other, and a mechanical spacer is interposed between the two silicon wafers. When the pressure is applied to the resultant structure, the silicon wafers start bonding together from a central portion. In the fusion bonding method or the SDB method, the surface cleanliness and roughness affect the bonding quality. The anodic bonding method needs to be implemented under the surface roughness of 1 μm or less, while the SDB method needs to be implemented under the surface roughness of 4 nm or less. As depicted by this condition, the surface roughness is one important factor in the SDB method.
The fusion bonding takes place in four steps. First, substrates are heated at room temperature until the substrates reach a temperature of 300° C. While the substrates are heated, oxygen (O2), hydrogen (H2), H2O, and hydroxyl (—OH) containing molecules are bonded together, producing hydrogen bonds between the substrates. This step is an initial bonding.
Second, the substrates are heated to a temperature of 700° C. from 300° C. While the substrates are heated, due to the dehydration, the H2O molecules are detached from the hydrogen bonds and diffused outside. At this stage, the hydrogen bonds are mainly formed by the —OH group containing molecules. Also, the elasticity of the substrates (e.g., silicon substrates) changes, and thus, non-contact portions (i.e., non-bonded portions) of the substrates start contacting with each other.
Third, the heating temperature goes up from 700° C. to 1,000° C. In addition to the H2O molecules, hydrogen molecules are detached from the hydrogen bonds and vigorously diffused outside. As a result, the bonding is directed towards a state in which the O2 molecules exist on a bonding interface (i.e., the interface between the substrates to be bonded). Also, the elasticity of the substrates changes more than before, thereby creating a strong bonding.
Fourth, the substrates are heated at a high temperature of 1,000° C. or more. Most of the atoms existing on the bonding interface disappear by diffusing into the inside or outside of the substrates (i.e., silicon). At the same time, the elasticity of the substrates changes, thereby providing the complete bonding.
Since the fusion bonding does not use a buffer or spacer usually interposed between the substrates as a medium, a high temperature treatment such as oxidation or diffusion can be performed. Since the bonding materials are substantially the same, their thermal coefficients are also the same. As a result, almost no thermal stress is exerted. The fusion bonding method can be effectively used in fabricating sensors and actuators based on silicon micromachining because various structures can be mechanically worked again by bonding the substrates (e.g., silicon) or performing several processes after the silicon bonding.
However, the fusion bonding is often sensitive to surface roughness, non-uniformity, and particles. In particular, the surface roughness needs to be controlled in an angstrom level. Due to this fact, even though the hermetic sealing is implemented based on strong covalent bonds formed through performing a high thermal annealing treatment at a temperature of 1,100° C., it is difficult to apply the fusion bonding method to semiconductor fabrication processes. A low temperature treatment with a precedent plasma treatment is being actively researched as one approach to overcome this difficulty.
According to a frit glass bonding (i.e., seal glass bonding) method, a glass powder including lead is mixed with a binder to obtain a paste. To achieve a bonding by using various methods such as a screen printer or an extrusion, the paste is placed over a region where MEMS processes proceed and bonded thereto at a temperature of 450° C. The frit glass bonding allows the hermetic sealing, and can be implemented to various vacuum devices. However, the frit glass bonding generally uses lead, which is not environmentally favorable, and occupies a large area in the wafer in addition to a chip area. Hence, the frit glass bonding method may not be implemented to the entire semiconductor fabrication process line.
According to an epoxy bonding (i.e., organic bonding) method, a high polymer such as polyimide or epoxy is used as a bonding medium. Thus, a low temperature treatment can be used in the epoxy bonding. However, the epoxy bonding method may not provide the hermetic sealing. Also, the epoxy boning method may have an aging effect due to a time factor, thereby resulting in a high chance of changing device characteristics. For these reasons, the epoxy bonding method is not currently considered for the implementation.
Among the above bonding methods, the anodic bonding method can provide an effect of using the same silicon wafers. However, the anodic bonding may provide a thermal degradation caused by a high thermal treatment. Accordingly, a wafer-level bonding method that can induce less stress caused by a thermal coefficient and be performed at low temperature needs to be developed to obtain reliable characteristics of MEMS devices.
In an attempt to achieve highly reliable MEMS device characteristics, a deep via formation method, a metal-metal bonding method, or a metal-silicon bonding method is proposed. However, the deep via formation method has the following disadvantages.
First, a bonding method based on the deep via formation and the metal-metal bonding may deteriorate sensors when a metal layer formed over a wafer for an MEMS sensor is etched to form a metal interconnection line. It may be difficult to control an etching for forming a deep via hole in a cap wafer responsible for packaging a top part. This difficulty is likely to affect uniformity in electroplating, further making it difficult to obtain a uniform bonding between the wafers. As a result, a small bonding margin may result, leading to a difficulty in obtaining intended yields. Also, since the cap wafer needs to be ground to a size of about 100 μm, a unit cost is expected to increase, while yields are expected to decrease.
Similar to the above bonding method based on the deep via formation and the metal-metal bonding, a bonding method based on the deep via formation and the metal-silicon bonding may have a difficulty in controlling the depth of a via hole in a cap wafer. A connection between a pad and a sensor wafer may be limited. Securing uniformity in bonding between wafers may be difficult. As a result, the wafers are less likely to be bonded together, thereby resulting in a decrease in yields. Also, the cap wafer needs to be ground to a size of about 100 μm in the bonding method based on the deep via formation and the metal-silicon bonding. Thus, a unit cost is expected to increase, while yields are expected to decrease.